Abrasive rotary cutting string manufacture and use

ABSTRACT

A method of producing an abrasive cutting string provides for coating a plastomeric string with a solution of a solvent and the plastomeric material of the string, applying an abrasive and removing the solvent to thereby provide a cutting string with excellent abrasive bind and string coverage, that can be manufactured at high speeds, and then used on a spool in the rotary cutting assembly of a string trimmer to incrementally feed at least one extremity of the abrasive string for rotation through a cutting swath.

The present application is a continuation of U.S. Ser. No. 11/640,029, filed Dec. 15, 2006.

FIELD OF THE INVENTION

This invention relates generally to an improved method of manufacturing abrasive cutting string commonly used as cutting string for cutting vegetation such as grass, weeds, brush and the like in combination with a rotary string trimmer with at least one extremity of the cutting string extending from a cutting head assembly and being rotated for impacting vegetation.

BACKGROUND OF THE INVENTION

It is known to use extruded thermoplastic string such as of Nylon to cut grass, weeds, brush and the like in combination with a rotary string trimmer. Conventional thermoplastic strings, however, suffer from several disadvantages. For example, conventional strings have a substantially smooth exterior surface. More particularly, the exterior surface of conventional strings is devoid of any abrasive or aggregate materials which produce cutting surfaces. Instead, conventional strings rely on the rotational velocity and impacting force of the string to cut weeds, grass, brush and the like. This, however, is an inefficient way to cut grass, weeds and particularly vegetation that cannot be felled with impacting force of the cutting string. In addition, the smooth surface of conventional strings is devoid of any air voids which cool the string when it is rotated. As a result, conventional strings are known to melt and fuse in the head of a rotary string trimmer under normal or high temperature conditions. This results in costly and time-consuming repairs and requires replacement of conventional strings.

Further, the cross-sectional shape of conventional strings is usually circular. Consequently, conventional strings do not have any edges or raised profiles which improve cutting efficiency, and instead rely on the rotational velocity and impacting force of the string to cut grass, weeds, brush and the like. As noted above, this provides mostly impacting force and very little cutting action. Still further, as a result of the high rotational velocity and composition of conventional strings, the strings are susceptible to damage and breaking, and thus they have a limited lifespan. As a result cutting strings are conventionally spooled within the cutting head assembly and lengths are advanced as needed to maintain an effective cutting swath for the extended extremity of the cutting string. Finally, because conventional strings are inefficient (e.g., they rely upon the rotational velocity and force of impact of the string rather than any cutting surfaces on the string), the strings impart undue stress upon the rotary string trimmer with which they are used.

Some attempts have been made to improve upon conventional circular strings used to cut grass, weeds, brush and the like in combination with a rotary string trimmer. For example, cutting strings have been extruded in different cross-sectional shapes. More particularly, the circular cross-sectional shape of conventional strings has been modified to a cross-sectional shape having one or more edges or raised profiles such as an octagon. This modification of conventional strings, however, also suffers from many of the same disadvantages as conventional strings of circular cross-section. While the impacting force of a raised profile string may be delivered more efficiently in comparison to conventional strings having a circular cross-sectional shape, conventional strings having a cross-sectional shape with raised profiles or edges still have a generally smooth lengthwise exterior surface. Consequently, conventional strings having a cross-sectional shape with raised profiles or edges still rely substantially on the rotational velocity and impacting force of the string to cut, and the result is an inefficient way to cut grass, weeds, brush and the like.

Further, like conventional smooth surface circular strings, conventional smooth surface strings having a cross-sectional shape with raised profiles or edges are susceptible to melting and fusion in the head of the rotary string trimmer under normal or high temperature conditions. Again, this results in costly and time-consuming repairs and replacement of the string. Still further, like conventional strings having a smooth, circular cross-section, conventional strings having a smooth, cross-sectional shape with raised profiles or edges have a limited lifespan due to the composition of the string and the high rotational velocity of the string. Finally, because conventional strings having edges or raised profiles are inefficient for cutting purposes inasmuch as they rely upon the rotational velocity and force of impact of the string rather than cutting surfaces on the string, they impart undue stress on the rotary string trimmer with which they are used.

In an effort to overcome these shortcomings, some attempts have been made to mix abrasive particles with thermoplastic resins to produce string. These attempts generally are very taxing on string extrusion equipment, tend to leave most of the abrasive particles buried in the string, and have abrasive particles even in the core of the string. The result is a relatively weak or breakable string without substantial external abrasive particle cutting surfaces. Alternatively, gluing abrasives to the string also results in added cost and difficulties in manufacturing and string performance.

A variety of techniques have been proposed to create a cutting string with an uncompromised core, and with abrasives only on the outer surface and periphery of the string. For instance Legrand, U.S. Pat. No. 6,630,226 proposes using an adhesive to bond abrasives to the surface of a thermoplastic resin string. Ledford and Wood, U.S. Ser. No. 11/006,024, proposes two techniques. One is heating the string so that it is soft enough to have abrasives embedded in the outer surface. However, heating the cutting string sufficiently for this embedding process weakens the string to the extent that it has too little strength to be used for rotary cutting. The second proposal of Ledford and Wood is to soften the exterior surface of the cutting string by exposure to a solvent such as formic acid, and then to apply abrasives to the softened string surface. However, softening the string surface is a relatively slow process requiring some length of time for formic acid to produce Nylon sufficiently softened that it may have abrasive embedded. The embedding techniques of either drawing the acid softened string through sand or depositing sand on the softened string before taking the string up on rollers also provide relatively poor abrasive coverage on the string surface. The processing speeds that can be obtained appear unsuitable for commercial manufacture, formic acid usage is high, and the amount of formic acid lost to evaporation is substantial.

In addition, while an abrasive cutting string should preferably maintain nearly the same strength as a conventional cutting string, because the thermoplastic or plastomeric abrasive string will also be subjected to repeated impacts while rotating at high speed, the extended extremity of the abrasive cutting string will inevitably be damaged or break. Therefore the abrasive cutting string must be suitable for placement upon a spool that can be mounted in a cutting head assembly for the incremental dispersing or advancement of at least one extremity of the abrasive cutting string as needed to maintain an effecting cutting swath.

Accordingly, it is an advantage of the invention described herein to provide an improved cutting string with an abrasive surface for cutting grass, weeds, brush and the like in combination with a rotary string trimmer.

It is a yet another advantage of the invention to provide a simple and inexpensive method for making such an improved cutting string used in a rotary string trimmer.

Additional advantages of the invention will become apparent from an examination of the drawings and the ensuing description.

SUMMARY OF THE INVENTION

The invention comprises a method of manufacturing cutting string for cutting grass, weeds, brush and the like when used in combination with motorized rotary head cultivation implements. The preferred cutting string is composed of a plastomeric material and the manufacturing process fastens abrasives in plastomeric materials coated on the periphery of the string. The abrasives are applied to the cutting string after the cutting string is coated with a solution of a solvent and the plastomeric material comprising the string. This results in a viscous periphery of the string in which abrasive particles are embedded. Upon curing of the plastomeric material as by evaporation of solvent, the abrasives are set about the periphery of the string in the plastomeric material comprising the cutting string.

The method comprises the steps of forming a plastomeric cutting string, which is usually extruded in the same way as conventional cutting strings in this field from a plastomeric material, typically polyamide. The cutting string may have various profiles, the simplest shape being a circular cross-section but numerous alternative profiles have been proposed for cutting string such as disclosed in Skinner, Des. 376,739, Des. 379,052, Des. 379,418 and Des. 379,419; Fogle, U.S. Pat. No. 6,434,837 and Morabit, U.S. Pat. No. 5,996,233 with various jagged profiles. In a preferred method the cutting string is then passed through a solution of about 10 to 20 percent weight by volume of a solution of plastomeric string material. This results in the string having a viscous periphery of plastomeric material to which an abrasive coating is applied. The abrasive particles are typically inorganic particles such as silica, glass, fine sand, emery, marble, and coarse garnet. A variety of metallic particles such as carborundum, zinc, iron, and aluminum, especially in the form of powdered oxides of appropriate particle size may also be suitable abrasive coatings. In addition, particles of synthetic materials such as rigid plastics and combinations of two or more of the previously mentioned substances may also be advantageous.

The addition of abrasive particles enhances the cutting efficacy of the cutting strings by providing not only the impacting force of the string but also providing a sawing effect. The combination of the abrasive sawing effect in combination with the impact of the cutting line due to the rotational velocity significantly improves the cutting capability of the line, especially in connection with the heavier vegetation that cannot be severed by impacting force alone. As an added benefit, the nature of the abrasive particles may also improve the thermal characteristics of the cutting line and reduce any tendency of the cutting line to stick due to approaching melting point of the plastomeric material. In addition, with the appropriate abrasive coatings it becomes possible to rely upon the cutting or sawing effect of the abrasives to a much greater degree than the impacting effect of the rotational velocity imparted by the rotating head of the implement. In this case, the rotational speed of the head may be significantly reduced to perhaps several hundred rpm rather than several thousand rpm facilitating the use of less powerful motors and reducing the noise generated by the rotary head cultivation implement employed.

BRIEF DESCRIPTION OF THE DRAWINGS

The presently preferred embodiments of the invention are illustrated in the accompanying drawings in order to facilitate an understanding of the invention, in which like reference numerals represent like parts throughout, and in which:

FIG. 1A is a partial cross-sectional view of a preferred embodiment of a cutting string made in accordance with the present invention.

FIG. 1B is an enlarged view of the circled portion of the cutting string illustrated in FIG. 1A.

FIG. 2 is a schematic diagram of a preferred embodiment of the method for making cutting string in accordance with the invention.

FIG. 3 is a sectional view of a prior art string trimmer spool mounted in a cutting head.

DESCRIPTION OF THE INVENTION

Referring now to the drawings, a cutting string 10 and a preferred method for making the cutting string of the invention are illustrated by FIGS. 1 through 2. As shown in FIG. 1A, a preferred cutting string is designated generally by reference numeral 10. The cutting string 10 is composed of a plastomeric material 11 such as Nylon 6 or Nylon 66 that is extruded to have a substantially circular cross-sectional shape. It is contemplated within the scope of the invention, however, that cutting string 10 may be composed of any suitable material that may be formed into a string-like configuration. Further, it is contemplated within the scope of the invention that cutting string 10 may be formed to have any suitable cross-sectional shape such as oval, crescent, semi-circular, triangular, square, rectangular, pentagonal, hexagonal, octagonal or any other polygonal shape. It is further contemplated within the scope of the invention that cutting string 10 may be produced by a process other than extrusion such as molding, drawing or any other suitable process for producing string.

Still referring to FIG. 1A, cutting string 10 includes a plurality of abrasives 12. Abrasives 12 are preferably composed of very fine foundry sand or some other similar material such as very fine aggregates, metal oxide powders, and the like. Abrasives 12 are attached to cutting string 10 by being partially embedded on the exterior surface of cutting string 10, so that feet 13 of abrasives 12 are surrounded by the plastomeric material 11 of the cutting string 10. Also in the preferred embodiment, abrasives 12 cover the substantially entire exterior surface of cutting string 10. Abrasives 12 are adapted to produce a non-smooth surface on the exterior of the cutting string. As shown in FIG. 1B, the non-smooth surface produced by abrasives 12 produces a plurality of cutting surfaces 14 protruding from the plastomeric material 11 of the cutting string 10. The plurality of cutting surfaces 14 improves the cutting efficiency of the cutting string 10 as compared to conventional smooth surface strings.

As a result of cutting surfaces 14, cutting string 10 need not be rotated at the same high velocity as conventional smooth surface strings in order to cut the same grass, weeds, brush and the like. Further, cutting string 10 having cutting surfaces 14 reduces the amount of stress imparted upon the rotary string trimmer with which it is used because of its improved cutting efficiency. Still further, the abrasives 12 on cutting string 10 permit the string to cut through larger, thicker and denser vegetation than conventional smooth surface strings.

Abrasives 12 on the surface of string 10 provide an additional advantage. As shown in FIG. 1B, the abrasives produce a non-smooth surface which results in a plurality of air voids 16. Air voids 16 cool cutting string 10 as it rotates, thereby reducing the temperature of the cutting string and inhibiting melting and fusion of the plastomeric material 11 of cutting string 10.

Referring now to FIG. 2, a presently preferred method of making cutting string 10 is illustrated. More particularly, as shown in FIG. 2, the method of making cutting string 10 includes the steps of feeding Nylon pellets into hopper 22, which are melted and extruded in the form of cutting string 10 by extruder 24. As discussed above, Nylon 6 or Nylon 66 any other suitable plastomeric material may be used to produce the cutting string. Further, as discussed above, cutting string 10 may be extruded by extruder 24 or formed by any other suitable method or device for producing a cutting string. A typical circular cross sectional string core would have a diameter between about 0.1 and 0.05 inches, and most preferably between 0.06 and 0.07 inches for use in a customary string trimmer application.

Still referring to FIG. 2, the illustrated method also includes the step of coating the cutting string 10 with a solution of the string core material for instance, a Nylon 6 string core will be passed through a bath 26 made in a ratio of about 15 grams of Nylon 6 per 100 milliliters of formic acid. A typical bath would be approximately 10 to 20 percent, and preferably about 13 to 16 percent, weight by volume of Nylon 6 or other plastomeric material comprising the cutting string 10 and the balance of one or more solvents. Lower concentrations of the dissolved plastomeric material are not desirable because it may result in an insufficiently viscous material on the periphery of the cutting string to bond with the adhesive particles, at least without slowing the process down substantially as required for the solvents to begin to dissolve the string itself to generate the material needed for adhesion. Higher concentrations of the plastomeric material would be desirable, however, depending upon the material used, it can be difficult to apply the coating of dissolved polymer evenly and to prevent the buildup of a greater thickness of additional plastomeric material than is desired. Preferably bath 26 is in communication with a fume hood and condenser 40 in order to capture solvent vapors from the bath 26. The cutting string 10 after passing through the solution of bath 26 has the excess solution brushed off to achieve relatively uniform coating over the cutting string 10. Brushes would typically be made of another material such as Nylon 12 that is not dissolved by the solvent, which in the case of Nylon 6 is most typically formic acid. The wet, solution-coated cutting string 10 proceeds directly through fine abrasives such as sand, coarse garnet, aluminum oxide or other metal powders as discussed previously. The string may be immersed in an abrasive or simply have an abrasive applied to it at this abrasive coating station 28. The abrasive covered string then proceeds to an evaporation station 30 where hot air may be circulated over and around the string to remove solvents, such as formic acid, which are captured by a vent hood and to condenser 36 so that formic acid may be precipitated and in some cases reused. The dried cutting string is preferably finished to a uniform size as by passing through sizing die 38 and then wound on a take up reel 32. It is contemplated within the scope of the invention that the cutting string may be stored in any suitable manner using any suitable storage device.

The selection of Nylon 6 and Nylon 66 as plastomeric materials utilized for the cutting string is desirable because of their solubility in formic acid. As a solvent, formic acid is desirable because the temperature required to evaporate the residual formic acid in heating chamber 30 is typically only between 100 and 150 degrees centrigrade. The solution of formic acid and Nylon 6 or Nylon 66 also has excellent tenacity so that the abrasive materials stick easily in, the viscous coating on the cutting string.

The abrasive densities achieved on the cutting strings manufactured in this fashion are much improved over the techniques of Ledford and Wood and the strength of the cutting string is not diminished. Continuous processing speeds between 200 and 2,000 feet per minute may be achieved on properly engineered manufacturing equipment. The preferred particle size for abrasives is between about 30 mesh and 70 mesh. When particle sizes of the abrasive material become larger than 30 mesh, it is more difficult for the particles to adhere to the viscous coating on the cutting string. It is also desirable that the abrasive particles have angular surfaces as round glass beads, while initially adhering, and simply fall off when the cutting string is rubbed. The majority of suitably angular particles may be securely fixed to the string, preferably with the majority of the abrasive particles having a bind strength to the string in excess of half of the tear strength of the beginning string core.

A representative coating density on a 0.06334 in diameter string core covered with 30 mesh sand is about 0.4 grams of sand per square inch of string core surface after the final uniform sizing of the abrasive string. The coating density for a 30-40 mesh ground garnet (abrasive) is 0.43-0.45 grams per square inch after final sizing. The resulting abrasive cutting string will typically have a diameter of between about 0.095 and 0.125 inches, and most preferably between 0.10 and 0.105 inches. It is desirable to achieve a coating of abrasive particles that cover at least 95, 96, 97, or 98 percent of the surface area of the string core, and preferably at least 99 percent of the surface area. The resulting abrasive string will also have good tensile strength preferably in excess of 30 pounds, 60 pounds, 90 pounds, 120 pounds, 150 pounds, 180 pounds and 210 pounds.

As shown in FIG. 3, a typical prior art rotary cutting assembly with cutting string feed is normally mounted on and driven by a suitable drive unit indicated generally at 110. The drive unit may be an electric or gasoline motor, the housing of which is coupled through a sleeve member 112 to a suitable support handle 115 for positioning the cutting assembly in the cutting of weeds. A suitable annular bumper 120 extends from the base of the housing and a drive shaft 122 extends therefrom to mount the rotary cutting assembly which is indicated generally at 125. The details of the drive assembly and the support or handle structure may take varying forms.

The rotary cutting assembly is comprised of a driven member having first and second plate members 130 and 132 respectively. Plate member 130 has a cylindrical hub 134 projecting from one surface thereof through which the drive shaft 122 extends. This surface of the plate member advantageously has a plurality of extending radial fins which, upon rotation, will provide cooling for the assembly. The plate member 130 has a second cylindrical hub portion 140 extending from the opposite surface with a cylindrical aperture therethrough concentric with the aperture of the hub portion 134 and receiving the extremity of the shaft 122. In addition, plate member 130, has a pair of elliptical apertures 42 therein and a plurality of posts 45 extending from the surface of the plate member opposite the radial fins. The plate member 130 is mounted on the drive unit 110 through the shaft 122 thereof. The shaft extends through the hub 134 and into hub 140 in a recessed portion 48 therein wherein a threaded extremity 49 of the drive shaft is located. A retainer flange 50 having an internal threaded surface or threaded recess is positioned in the recessed portion 48 of the hub 140 and threaded onto the threaded extremity of the shaft with a collar portion 52 bearing against the end of the hub 140 to retain the hub and the plate member 130 on the end of the shaft.

The hub 140 has an outer peripheral surface with an annular flange portion adjacent the plate member 130 which portion, as will be later noted, serves as a guide surface. The lower extremity of the hub is recessed from this guide surface and a plurality of triangular shaped teeth members 60 and 62 are distributed about the periphery of the same. Teeth members 60 are distributed along a common plane adjacent the annular flange guide surface and have a height equal to the external dimension or surface of the guide surface of the hub. These teeth or ratchet means are angularly spaced about the peripheral surface of the hub and the number of teeth may be varied, for purposes of determining the amount of cutting string to be incrementally fed from the storage spool within the driven member. A second set of teeth 62 are disposed in a plane parallel to and spaced from the plane of the teeth 60 and angularly distributed so that they are positioned radially in between the teeth 60 and equidistantly spaced having the same angular spacing peripherally on the hub.

The outer plate member 132, has a pair of post members 70 projecting therefrom with notched surface 72 in the extremity of the same. Plate member 132 has a central aperture 75 therein with an external dished surface 76 leading to the same. The posts 45 of plate member 130 have wear sleeves positioned over the same which act as spacers to space the plate member 130, 132 apart with the posts 70 having the notched extremities 72 extending through the apertures 42 in the plate member 130 and locking the same to the plate member 130. The plate member 132 around the post members 70 is slightly deformable such that the notched surfaces may be moved into and out of the apertures 42 for assembly and release of the plate members. These plate members of the driven member define a spaced area therebetween in which there is positioned the spool 80 which mounts the cutting string 85 of the rotary cutting assembly. Although not specifically shown, one or more strands of cutting string may be wound on the spool and secured to the hub of the same with the opposite extremity or extremities extending from the spool to provide the cutting surface conventional with a rotary cutting assembly of this type. Where two or more such lines are used, they will be wound in the same direction, but coming out through the posts 45 equidistantly spaced such that upon a predetermined relative rotation of the spool to the driven member, the lines will be unwound from the spool and extend out of the driven member. Except for the posts 45 and 70 the area between the plates 130 and 132 is open to expel dirt from the housing through centrifugal force. The plate 132 extends beyond the posts 45 and 70 to form a lip 79 which prevents the line from shearing off near the posts.

As will be seen in FIG. 3 the spool 80 is formed in a conventional manner with a pair of spaced sides 82 held together by a common hub portion and in the illustrated embodiment, has a hemispherically shaped hub member 90 formed integral therewith and projecting from one side of the spool. The interior of the hub portion of the spool has a plurality of projecting triangular teeth members 92 distributed about the inner periphery of the same. The interior of the hub has a recessed shoulder portion 95 which fits over the end of the hub 140 with the retaining flange 50 thereon to be guided thereon for longitudinal movement of the spool in an axial direction relative to the hub 140. Positioned between the interior of the hub 90 and the retainer flange 50 is a spring member 100 which fits into a recessed surface 51 in the retaining flange 50 and into a recessed surface 101 in the top of the hub 90. The spring is a compression spring which will bias the hub and hence, the spool 80 formed integral therewith, relative to the hub 140 such that the surface of the spool will bear against the plate 132. However, when the hub 90 is engaged by a surface, such as by depressing the hub against a solid surface, the spring 100 will be compressed allowing the hub 90 with the spool 80 integral therewith to slide on the hub 140 through the guiding surfaces on the respective hubs to permit the teeth 92 to disengage from the row of teeth 62 on the hub 140. With the driven member rotating, a relative rotation will take place between the spool 80 and the hub 140 or the parts of the driven member allowing the spool to rotate until the flat surfaces of the teeth 92 next to adjacent teeth 60 are engaged. Once the pressure on the spring is released by withdrawal of the hub 90 from the solid surface, the compression of the spring will move the spool 80 back towards the plate member 132 disengaging the teeth 92 from the teeth 60 and allowing them to engage with the teeth 62 on the hub 140. This will allow another step of relative rotation between the spool and the driven member, the purpose of which is to unwind an increment of cutting string from the spool and allow the free extremity of the cutting string to extend out of the confines of the driven member between the posts 45 or 70 to provide a longer cutting surface for the rotary cutting assembly. This will replace any portion of the cutting string which has been broken off due to wear, fatigue, or impact with solid objects in the cutting operation.

Whenever it is desired to disassemble the driven member for replacement of the spool, the posts 70 will be deflected, releasing the notched surfaces 72 from the sides of the apertures 42 in the plate member 130 and permitting separation and removal of the plate member 132 therefrom. Thereafter, the spool 80 and the hub 90 integral therewith will be lifted off of the hub 140 and new cutting line may be mounted thereon or a replacement unit with cutting string thereon may be inserted in its place. Thus, no separate or special tools will be required for assembly or dissasembly of the driven member for replacement of the spool or the addition of cutting string thereon. The posts 45 and 70 also serve as a supporting surface for the free end of the line from the spool which is extended by centrifugal force upon rotation of the head directing the cutting string out from the driven member between the posts in a conventional manner. Where the extended portion of a cutting string is broken upon impact or fatigue, the user of the rotary cutting assembly while rotating need only press the entire assembly against a surface causing the hub through its contact with the surface to overcome the compression of the spring 100 and move the spool assembly out of engagement with one set of ratchet teeth and into engagement with the other. Upon lifting or releasing of the rotary cutting assembly away from the surface, the spring will return the spool assembly within the driven member and against the plate member 132 allowing another increment of rotary movement between the spool assembly and driven member to complete the advance or automatic feed of an increment of cutting string from the spool through the action of centrifugal force on the line rotating the spool relative to the housing

In operation, several advantages of the apparatus and method of the invention are achieved. The abrasives of the cutting strings produce a plurality of cutting surfaces and air voids about the exterior surface of the string. The irregular exterior surface of these cutting strings results in several advantages. For example, the plurality of cutting surfaces produced by the abrasive particles on the cutting string improves the cutting efficiency of the string. More particularly, the cutting surfaces of the cutting string contribute to the ability of the string to cut grass, weeds, brush and the like. Unlike conventional smooth surface cutting strings which rely almost solely upon the rotational velocity and force of impact of the string to cut grass, weeds, brush and the like, the abrasive cutting string of the invention relies on the cutting surfaces of the embedded abrasives as well as the rotational velocity and impacting force of the cutting string to cut vegetation. The result of the plurality of cutting surfaces on the periphery of the cutting string is that the cutting efficiency of the abrasive cutting string is increased, the rotational velocity of the abrasive cutting string need not be as high as the rotational velocity of a conventional smooth surface cutting string in order to cut the same grass, weeds or the like, the amount of stress imparted upon the rotary string trimmer in which the cutting string is used is decreased, the cutting string cuts grass, weeds and the like up to 40% faster than conventional smooth surface strings, and the abrasive cutting string is capable of cutting grass, weeds and the like that are up to ten times larger than the vegetation that conventional smooth surface cutting strings are capable of cutting.

The plurality of air voids produced by the abrasives of the abrasive cutting string also produce several advantages. For example, the air voids of the preferred cutting string have a cooling effect upon the string as it is rotated such as by a rotary string trimmer. The cooling effect of the air voids reduces the likelihood that the cutting string will melt or fuse under high temperature conditions. Consequently, repairs and replacements of the cutting string are reduced as compared to conventional smooth surface cutting strings. As a result, the cutting string is less expensive and less time-consuming to repair and requires less frequent replacement.

The methods of making the abrasive cutting string also achieve several advantages. For example, the described method provides a simple and inexpensive way to manufacture the preferred cutting string.

All publications, patents and patent documents are incorporated by reference herein as though individually incorporated by reference. Although preferred embodiments of the present invention have been disclosed in detail herein, it will be understood that various substitutions and modifications may be made to the disclosed embodiment described herein without departing from the scope and spirit of the present invention as recited in the appended claims. 

I claim:
 1. A method of manufacturing an abrasive cutting string for use in a rotary string trimmer comprising the steps of: (a) selecting a string made of a plastomeric material; (b) coating the string with a solution comprising a solvent and the plastomeric material; (c) applying an abrasive to the solution coated string; (d) removing the solvent from the abrasive coated string; (e) placing the abrasive coated string upon a spool in a cutting head assembly of a rotary string trimmer with at least one extremity of the abrasive coated string extending for rotation through a cutting swath.
 2. The method of claim 1 further comprising the step of winding the abrasive cutting string upon a reel before placing the abrasive cutting string on the spool.
 3. The method of claim 1 wherein the plastomeric material is polyamide.
 4. The method of claim 3 wherein the polyamide is selected from Nylon 6, Nylon 66 and mixtures thereof.
 5. The method of claim 4 wherein the solvent is formic acid.
 6. The method of claim 4 wherein the polyamide is in the solution at a concentration of between 10 percent and 20 percent weight by volume.
 7. The method of claim 1 wherein after solution is applied to the string the excess solution is brushed off the surface of the string.
 8. The method of claim 1 wherein solvent vapors are captured when the solvent is removed and the solvent is recovered by condensation.
 9. The method of claim 1 wherein the abrasive is selected from the group of silica, glass, fine sand, emery, marble, coarse garnet, metal particles, plastic particles, and mixtures thereof.
 10. The method of claim 1 wherein the abrasive particles have a mesh size in the range from about 30 mesh to about 70 mesh.
 11. The method of claim 1 wherein the string is coated with abrasive particles at a speed of at least 200 feet per minute.
 12. The method of claim 1 wherein the abrasive particles cover at least 99 percent of surface area of the string.
 13. The method of claim 1 wherein at least 50 percent of the abrasive particles have a bind strength in excess of half of the tear strength of the string of plastomeric material.
 14. The method of claim 1 wherein the diameter of the uncoated string of plastomeric material is greater than 60 percent of the diameter of the abrasive coated string.
 15. The method of claim 1 wherein the abrasive is applied to the solution coated string at a rate of about four grams per square inch of surface on the uncoated string of plastomeric material.
 16. The method of claim 1 wherein the abrasive particles cover at least 95 percent of the surface areas of the string and at least half of the abrasive particles have a bind strength greater than half the tear strength of the string's plastomeric material.
 17. A method of manufacturing an abrasive cutting string for use in a rotary string trimmer comprising the steps of: (a) selecting a string made of a Nylon soluble in formic acid; (b) coating the Nylon string with a solution of 10 to 20 percent by weight of Nylon dissolved in formic acid; (c) applying abrasive particles of between about 30 mesh and 70 mesh size to the solution coated Nylon string; (d) heating the abrasive coated string to a temperature of between about 100 and 150 degrees centigrade to substantially remove the formic acid; (e) recovering formic acid by condensation; (f) uniformly sizing the abrasive string; and (g) winding the abrasive coated string upon a reel.
 18. The method of claim 17 where the abrasive particles cover over 95 percent of the surface of the string made of a Nylon.
 19. The method of claim 17 wherein the diameter of the string made of Nylon is greater than 60 percent of the diameter of the abrasive coated string.
 20. The method of claim 17 wherein the abrasive is applied to the solution coated Nylon string at a rate of about four grams per square inch of surface of the string made of Nylon. 